There are numerous examples of out of range alarm systems. Such systems typically include a parent unit and one or more, portable, child units adapted to be worn by a person. The child units typically transmit an RF signal which is received by the parent unit. The distance between the units is determined based on the strength of the signal received from a child unit. These systems often include some means of generating an alarm when the distance between the units exceeds a predetermined amount, as a means of notifying the monitoring personnel at the parent unit when the animal or person, or persons, wearing the child units are out of range.
These systems have proved unreliable because they rely on received signal strength as a means of determining distance. This calculation presumes that the signal will always emanate at the same strength, and that it will be attenuated as a function of distance in the same manner, regardless of the strength of the batteries used to power the child units, the physical surroundings of the area in which the system is used, the positions of the transmit and receive antennas, and weather conditions and other factors which cause differences in signal attenuation. Because there are always unknown factors which will, almost certainly, cause RF signals to attenuate differently, such systems are inherently unreliable.